The history of the exclusion principle is already an old one, but its conclusion has not yet been written. It is now eighty years since Wolfgang Pauli introduced an ‘extremely natural’ prescriptive rule, while dealing with some spectroscopic anomalies that beset physicists in the heyday of the old quantum theory. The rule excluded the possibility that any two bound electrons in an atom were in the same dynamic state, identified by a set of four quantum numbers. Hence the name of Ausschlieβungsregel (exclusion rule), or Pauli's Verbot (Pauli's veto) as Werner Heisenberg nicknamed it. The far-reaching physical significance of this rule became clear only later. From spectroscopy to atomic physics, from quantum field theory to high-energy physics, there is hardly another scientific principle that has more far-reaching implications than Pauli's exclusion principle. It is thanks to Pauli's principle that one obtains the electronic configurations underlying the classification of chemical elements in Mendeleev's periodic table as well as atomic spectra. To this same principle we credit the statistical behaviour of any half-integral spin particles (protons, neutrons, among many others) and the stability of matter. Shifting to high-energy physics, it is the exclusion principle that fixes the crucial constraint for binding quarks in hadrons, which together with leptons compose our physical world. This book advances a philosophical analysis of the enduring and far-reaching validity of Pauli's principle. It does not aim to address what a scientific principle is.
The history of the exclusion principle is already an old one, but its conclusion has not yet been written. It is now eighty years since Wolfgang Pauli introduced an ‘extremely natural’ prescriptive rule, while dealing with some spectroscopic anomalies that beset physicists in the heyday of the old quantum theory. The rule excluded the possibility that any two bound electrons in an atom were in the same dynamic state, identified by a set of four quantum numbers. Hence the name of Ausschlieβungsregel (exclusion rule), or Pauli's Verbot (Pauli's veto) as Werner Heisenberg nicknamed it. The far-reaching physical significance of this rule became clear only later. From spectroscopy to atomic physics, from quantum field theory to high-energy physics, there is hardly another scientific principle that has more far-reaching implications than Pauli's exclusion principle. It is thanks to Pauli's principle that one obtains the electronic configurations underlying the classification of chemical elements in Mendeleev's periodic table as well as atomic spectra. To this same principle we credit the statistical behaviour of any half-integral spin particles (protons, neutrons, among many others) and the stability of matter. Shifting to high-energy physics, it is the exclusion principle that fixes the crucial constraint for binding quarks in hadrons, which together with leptons compose our physical world. This book advances a philosophical analysis of the enduring and far-reaching validity of Pauli's principle. It does not aim to address what a scientific principle is.
The exclusion principle was the final outcome of Pauli's struggle to understand some spectroscopic anomalies in the early 1920s: doublets were observed in the spectra of alkali metals, singlets and triplets in the spectra of the alkaline earths, and even more anomalous patterns were observed when chemical elements were placed in an external magnetic field (anomalous Zeeman effect and Paschen–Back effect). These anomalous spectra challenged the old quantum theory, and prompted a radical theoretical change (Section 2.1). From 1920 to 1924 Alfred Landé, Werner Heisenberg, and Niels Bohr were all engaged in trying to save the traditional spectroscopic model (the so-called atomic core model) and to reconcile it with the observed anomalies. The impasse was solved only with Pauli's introduction of a fourth degree of freedom for the electron, and the consequent demise of the atomic core model (Section 2.2). What Pauli called the ‘twofoldness’ [ Zweideutigkeit ] of the electron's angular momentum was soon reinterpreted as the electron's spin (Section 2.3). Pauli's exclusion rule was announced in this semi-classical spectroscopic context that characterized the revolutionary transition from the old quantum theory to the new quantum theory around 1925. The prehistory of Pauli's exclusion principle Atomic spectra and the Bohr–Sommerfeld theory of atomic structure The existence of spectral lines had been known to scientists since the beginning of the nineteenth century when Wollaston and Fraunhofer first observed the dark absorption lines in the spectrum of the Sun.
Shortly after their introduction, the electron's Zweideutigkeit and Pauli's Ausschliessungsregel were embedded into a growing theoretical framework: from Fermi–Dirac statistics in 1926 (Section 4.2), to the non-relativistic quantum mechanics of the magnetic electron with Pauli's spin matrices in 1927 (Section 4.3); from Wigner and von Neumann's group theoretical derivation of the spin matrices in 1927 (Section 4.4), to Jordan's reinterpretation of Pauli's exclusion principle in terms of anticommutation relations for particle creation and annihilation operators in 1928 (Section 4.5), which paved the way for quantum field theory. The most important step in building up this theoretical framework was the transition from non-relativistic to relativistic quantum mechanics, with Dirac's equation for the electron in 1928 (Section 4.6), which finally allowed the derivation of the electron's spin with its anomalous magnetic moment and hence clarified in a conclusive way the origin of spectroscopic anomalies. The negative energy solutions of the Dirac equation, and Dirac's attempt to accommodate them via the hole theory in 1930, anticipated the experimental discovery of the antiparticle of the electron, the positron. Sections 4.7-4.9 reconstruct Pauli's sustained criticism of Dirac's hole theory. The history of this debate is intertwined with Pauli's search for a spin–statistics connection, which finally culminated in the spin–statistics theorem in 1940. With this theorem, the nomological shift from the status of a phenomenological rule to that of a fundamental scientific principle is finally completed. Introduction The year 1925 was the annus mirabilis for quantum mechanics.
The aim of this book was to investigate the rationale for Pauli's principle, and the conditions under which we are justified in regarding a phenomenological and contingent rule as an important scientific principle. To this purpose, I urged a Kantian perspective. As a conclusion, I want to foreshadow how this perspective relates to contemporary discussions about images of science, and how it bears upon the on-going debate on scientific realism in philosophy of science. I shall barely scratch the surface of this complex topic and its vast literature. The best I can do in these concluding remarks is to suggest a philosophical position that I think deserves to be further explored. What follows should then be read more as an outline for future research, than as a conclusion. I began this book by reconstructing the origins of the exclusion principle as a phenomenological rule arising from the spectroscopic research of the 1920s. I argued that the rule was derived – in conjunction with the concept of the electron's Zweideutigkeit – from spectroscopic phenomena with the help of some theoretical assumptions, in the period of revolutionary transition from the old to the new quantum theory. I defended the prospective intelligibility of this revolutionary transition in the light of the piecemeal process of transformation of the old quantum theory, from which brand new scientific concepts and nomic generalizations followed.
This is the first scientific book devoted to the Pauli Exclusion Principle, which is a fundamental principle of quantum mechanics and is permanently applied in chemistry, physics, molecular biology and in physical astronomy. However, while the principle has been studied for more than 90 years, rigorous theoretical foundations still have not been established and many unsolved problems remain. Following an introduction and historical survey, this book discusses the still unresolved questions around this fundamental principle. For instance, why, according to the Pauli Exclusion Principle, are only symmetric and antisymmetric permutation symmetries for identical particles realized, while the Schrödinger equation is satisfied by functions with any permutation symmetry? Chapter 3 covers possible answers to this, while chapter 4 presents effective and elegant methods for finding the Pauli-allowed states in atomic, molecular and nuclear spectroscopy. Chapter 5 discusses parastatistics and fractional statistics, demonstrating that the quasiparticles in a periodical lattice, including excitons and magnons, are obeying modified parafermi statistics. With detailed appendices, The Pauli Exclusion Principle: Origin, Verifications and Applications is intended as a self-sufficient guide for graduate students and academic researchers in the fields of quantum mechanics, theoretical chemistry, physics, and applied mathematics. It will be a valuable resource for any reader interested in the foundations of quantum mechanics and its applications in areas such as atomic and molecular spectroscopy, spintronics, molecular biology, and applied fields of quantum information.
Despite the success of Schrödinger’s description of the H-atom, it became apparent that the spectrum of the simplest multi-electron atom—helium—could not be so readily explained. And the spectra of other atoms showed ‘anomalous’ splitting in a magnetic field. In 1920 Sommerfeld introduced a fourth quantum number. A few years later Pauli was led to the inspired conclusion that the electron must have a curious ‘two-valuedness’ characterized by a quantum number of ½, and went on to discover the exclusion principle. Perhaps this is because the electron possesses a self-rotation, leading to the notion of electron spin, potentially explaining why each orbital can accommodate only two electrons. Heisenberg traced this behaviour back to the symmetry properties of the wavefunctions. By observing which transitions in the spectrum of helium are allowed and which are forbidden, we can deduce the generalized Pauli principle, from which the exclusion principle follows.
An easy-to-understand guide to the complex subject of quantum physics. Quantum physics is how scientists describe the world of the very small. For other people, however, the rules of quantum physics seem to violate all logic: How can a particle be in more than one place at the same time? How can it tunnel through an impenetrable barrier? How can a cat in a box be both alive and dead? This book explains the complexities of quantum physics in bite-sized "lessons" that make it clear and accessible to all readers. The sections and chapters are: 1. Atoms -- quantum; quantum physics in everyday life; the periodic table; atoms and nuclei; isotopes; hydrogen atom (energy levels and spectra) 2. Photons -- photoelectric effect; thermal emission and the Planck distribution; wave particle duality (Young's slit experiment) 3. Quantum devices -- superconductors; transistor, diode; light-emitting diode; laser 4. Spin -- spin; fermions; exclusion principle; Fermi Dirac distribution; Bose-Einstein statistics 5. Wave Mechanics -- Heisenberg uncertainty principle; wave functions; standing waves; wave particle duality 6. Subatomic Particles -- radioactivity; atomic particles; protons; neutrons; electrons 7. Particle Physics -- elementary particles; quarks; leptons; electron and electron neutrino; muon and muon neutrino; tauon and tau neutrino; bosons; photon; W and Z boson; gluon; Higgs boson; composite particles; hadrons; matter-anti-matter 8. Quantum Physics in Medicine and industry -- magnetic resonance imaging; superfluidity; cancer treatment by radiation 9. Into the Future -- quantum physics of space; quantum gravity; graviton; Hawking radiation (grand unified theory); dark matter; strings; dark energy. Throughout the book, there are timelines and profiles about the scientists who developed the theories and made important discoveries in quantum physics, including Dmitri Rutherford, Albert Einstein, Heike Kamerlingh Onnes, William Shockley, Isama Akasaki, Stephen Hawking, Enrico Fermi, and Max Born.